• NoamLewis

Noam Lewis

Email: <jones.noamle AT SPAMFREE gmail DOT com>

I figured a convenient way to experiment with Open CV using Python (on Windows): Since Python 2.5, the ctypes package is included, which allows you to access shared libraries (in Windows - DLLs) and call functions from them.

I used this package to quickly build a simple application with OpenCV's HighGui: The main advantage is that you don't need to compile anything - you simply use the supplied DLL's that you get with the Open CV package.

import ctypes

highgui = ctypes.cdll.LoadLibrary(r'C:\Program Files\OpenCV\bin\highgui100.dll')
# Other dlls you might wanna use:
# cv = ctypes.cdll.LoadLibrary(r'C:\Program Files\OpenCV\bin\cv100.dll')
# cxcore = ctypes.cdll.LoadLibrary(r'C:\Program Files\OpenCV\bin\cxcore100.dll')

class Window(object):
    def __init__(self, name="Window"):
        self.name = name
        highgui.cvNamedWindow(name, 1)

    def move(self, x, y):
        highgui.cvMoveWindow(self.name, x, y)

    def show_frame(self, frame):
        highgui.cvShowImage(self.name, frame)
        
class Capture(object):
    capture = None
    def __init__(self):
        raise NotImplementedError("Virtual class")
    
    def query_frame(self):
        return highgui.cvQueryFrame(self.capture)

    def __del__(self):
        highgui.cvReleaseCapture(self.capture)
        
class CameraCapture(Capture):
    def __init__(self, device = -1):
        self.capture = highgui.cvCreateCameraCapture(-1)

def wait_key(delay = 0):
    return highgui.cvWaitKey(delay)

def handle_events():
    wait_key(1)

And now:

capture = CameraCapture()
window = Window("Test")
while True:
    frame = capture.query_frame()
    window.show_frame(frame)
    handle_events()

Note

1. CTypes does NOT automatically supply type information for functions or structures (unlike CInvoke, which is yet to be released...)

• So if you want (or need) to access the internal structures represented by the pointers you receive, you'll have to manually declare them, like this example:

class cvSize(ctypes.Structure):
    _fields_ = [('width', ctypes.c_int),
                ('height', ctypes.c_int),
                ]
    
class IplImage(ctypes.Structure):
    _fields_ = [('nSize', ctypes.c_int),
                ('ID'   , ctypes.c_int),
                ('nChannels', ctypes.c_int),
                ('alphaChannels', ctypes.c_int),
                ('depth', ctypes.c_int),
                ('colorModel', ctypes.ARRAY(ctypes.c_char, 4)),
                ('channelSeq', ctypes.ARRAY(ctypes.c_char, 4)),
                ('dataOrder', ctypes.c_int),
                ('origin', ctypes.c_int),
                ('align',  ctypes.c_int),
                ('width',  ctypes.c_int),
                ('height', ctypes.c_int),
                ('roi',    ctypes.c_int), # actually ptr
                ('maskROI', ctypes.c_int),
                ('imageId', ctypes.c_int),
                ('tileInfo', ctypes.c_int),
                ('imageSize', ctypes.c_int),
                ('imageData', ctypes.c_char_p),
                ('widthStep', ctypes.c_int),
                ('BorderMode', ctypes.ARRAY(ctypes.c_int, 4)),
                ('BorderConst', ctypes.ARRAY(ctypes.c_int, 4)),
                ('imageDataOrigin', ctypes.c_char_p),
                ]

2. CTypes will not supply you with #define'd values - my quick, dirty and simple approach was to parse the .h files and create a python module that contains all the defines.

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